Network initiated handovers may occur to balance network load across multiple base stations (BSs) in the network, to perform maintenance (e.g., when a serving base station (SBS) needs to be removed from service for software or hardware upgrades), or for various other reasons. This is preferable from an operator and MS perspective for active calls. It is desirable for the selection of a target base station (TBS) for a mobile station (MS) to handover to depend on how well the MS is receiving signaling from that and other prospective TBSs. However, often the network doesn't have the most reliable information regarding the best handover target BS for an MS at a given instance of time.
A controlled handover in a WiMAX network refers to a handover where an MS hands off to a TBS which was previously notified and prepared by the serving BS (SBS) for an incoming handover by the MS. Controlled handovers in WiMAX and 802.16 based networks may be initiated by either the MS, SBS or Access Service Network (ASN) in response to deteriorating wireless conditions, network load balancing, network maintenance, etc. See e.g., IEEE 802.16Rev2/D2 section 6.3.22.2.2 and WiMAX Networking Specification Stage-3 Text R1.0 (v1.2), Section 4.7.2.
In a controlled handover, the network notifies prospective TBS candidates via handover preparation procedures so that they are prepared to receive a handover from the MS. Handover preparation of potential TBSs helps to reduce data loss during handover. It also enables the TBS to offer priority access to the MS when it contacts the TBS for handover, thereby reducing handover latency delays.
Handover preparation includes passing network context information such as the anchor authenticator ID, the anchor data path ID, and the latest MAC context information from the SBS to the TBSs. Handover preparation may also include the establishment of bearer connections between the TBS and network in order to continue the mobile's IP session.
In a network initiated controlled handover, the network proposes one or more candidate TBSs for an MS to handover to in the IEEE 802.16 MOB_BSHO-REQ message. See e.g., P802.16Rev2/D2 section 6.3.22.2 and WiMAX R1.2 Networking Specification Stage-3 Text, Section 4.7.2.1.6. The MS responds by sending a MOB_HO-IND message indicating its acceptance or rejection of the handover target BS(s) proposed by the serving BS. If the MS accepts a TBS proposed by the serving BS, it moves to the selected target BS and begins ranging to complete the handover. If the MS rejects the target BS(s) offered by the serving BS, it sets the T42 timer and waits for the SBS to propose an alternate TBS for the MS to handover to. This process repeats until an acceptable TBS for handover is offered by the serving BS or the T42 timer expires. See P802.16Rev2/D2 FIG. 150.
FIG. 4 is a signaling flow diagram that depicts a first example of a network initiated handover, in accordance with the prior art. The signaling flow of diagram 400 depicts a network initiated handover procedure for an 802.16 compliant MS similar to that described in NWG R1.0 v1.2 stage-3 section 4.7.2 and 4.7.2.1.6.
In diagram 400 the SBS notifies and prepares three candidate TBSs for the MS to handover to before requesting the MS to handover to one of them. The MS selects one of the candidate TBSs (candidate TBS-1) offered by the SBS. The TBSs that weren't selected (candidate TBS-2 and candidate TBS-3) by the MS are notified via network handover cancellation procedures. If the mobile rejects all the TBSs offered by the SBS for handover, the network not only has to notify the previous candidate TBSs prepared for the handover, but must also notify and prepare one or more new TBSs for a potential incoming handover from the MS. See e.g., IEEE 802.16 FIGS. 149 and 150.
As can be seen from diagram 400, handover preparation is expensive in terms of the network signaling required to notify prospective TBSs and allocating resources at these TBS to support a potential handover. After a TBS offered by the SBS is selected by the mobile, all of the other candidate TBSs previously notified and prepared for a potential handover, but then rejected, must be notified by the SBS. They are notified that they were not selected so that the resources they have allocated to support a potential handover may be released.
FIG. 5 is a signaling flow diagram that depicts a second example of a network initiated handover, in accordance with the prior art. The signaling flow of diagram 500 depicts network initiated handover procedures for an 802.16 compliant MS in which the mobile rejects all of the TBSs offered by the SBS. After the MS rejects the TBSs offered by the SBS, the SBS/ASN must come up with a new TBS to offer the MS. However, this requires guesswork on the part of the network. The example of diagram 500 shows the network offering candidate TBS-4 to the MS after candidate TBSs 1, 2 and 3 are rejected by the MS. In this example, the MS accepts candidate TBS-4; however, it could also have rejected candidate TBS-4 and continued rejecting candidate TBSs for awhile. Such a cycle of rejections may continue until the network eventually offers a candidate TBS acceptable to the MS or for some other reason the negotiation procedure ends. Neither of these are desirable outcomes, especially when the SBS/network is performing load balancing or the SBS is being taken out of service for some reason.
Thus, in view of the inefficiencies present in current network initiated handover techniques, new techniques that are able to improve network initiated handover are clearly desirable for advancing the art.
Specific embodiments of the present invention are disclosed below with reference to FIGS. 1-3. Both the description and the illustrations have been drafted with the intent to enhance understanding. For example, the dimensions of some of the figure elements may be exaggerated relative to other elements, and well-known elements that are beneficial or even necessary to a commercially successful implementation may not be depicted so that a less obstructed and a more clear presentation of embodiments may be achieved. In addition, although the signaling flow diagrams and/or the logic flow diagrams above are described and shown with reference to specific signaling exchanged and/or specific functionality performed in a specific order, some of the signaling/functionality may be omitted or some of the signaling/functionality may be combined, sub-divided, or reordered without departing from the scope of the claims. Thus, unless specifically indicated, the order and grouping of the signaling/functionality depicted is not a limitation of other embodiments that may lie within the scope of the claims.
Simplicity and clarity in both illustration and description are sought to effectively enable a person of skill in the art to make, use, and best practice the present invention in view of what is already known in the art. One of skill in the art will appreciate that various modifications and changes may be made to the specific embodiments described below without departing from the spirit and scope of the present invention. Thus, the specification and drawings are to be regarded as illustrative and exemplary rather than restrictive or all-encompassing, and all such modifications to the specific embodiments described below are intended to be included within the scope of the present invention.